Titanium Alloy Parts Milling

In many aerospace applications, titanium and its alloys are replacing traditional aluminum alloys. Today, the aerospace industry consumes about 42% of the world's total titanium production. From now until 2010, the demand for titanium materials is expected to continue to grow at a double-digit rate. New generation aircrafts need to make full use of the properties provided by titanium alloys. Both the commercial and military markets are pushing for titanium alloys. Boeing 787, Airbus A380, F-22 Raptor, F-35 Joint Strike Fighter (also known as Lightning II) and other new models have used a lot of titanium alloy materials.

The advantages of titanium alloy materials

Titanium alloys have high strength, high fracture toughness, and good corrosion resistance and weldability. With the increasing use of composite structures in aircraft fuselage, the proportion of titanium-based materials used in the fuselage will also increase, because the combined performance of titanium and composite materials is far superior to aluminum alloys. For example: Compared with aluminum alloys, titanium alloys can increase the life of the fuselage structure by 60%.
The extremely high strength/density ratio (up to 20:1, ie weight reduction of 20%) of titanium alloys provides a solution for reducing the weight of large components, which is a major challenge for aircraft designers. In addition, the inherently high corrosion resistance of titanium alloys (compared to steel) can save the cost of daily operations and maintenance of aircraft.

Need more processing capacity

Because it is more difficult to process than ordinary alloy steels, titanium alloys are generally considered to be difficult to machine materials. The removal rate of metal from a typical titanium alloy is only about 25% of that of most common steels or stainless steels, so processing a titanium alloy workpiece takes about four times as much as machining steel.
In order to meet the ever-increasing demand for titanium alloy processing in the aerospace industry, manufacturers need to increase production capacity, and therefore need to better understand the effectiveness of titanium alloy processing strategies. The processing of a typical titanium alloy workpiece starts from forging until 80% of the material is removed to obtain the final workpiece shape.
With the rapid growth of the aerospace component market, manufacturers have felt uncomfortable, coupled with the increased processing requirements due to the lower processing efficiency of titanium alloy workpieces, resulting in a significant tension in the processing capacity of titanium alloys. Some leading companies in the aviation manufacturing industry even openly question whether the existing machining capacity can complete the task of processing all new titanium alloy parts. Since these workpieces are usually made of new alloys, the machining methods and tool materials need to be changed.

Titanium alloy Ti-6Al-4V

Titanium alloys have three different structural forms: alpha titanium alloys, alpha-beta titanium alloys, and beta titanium alloys. Commercially pure titanium and alpha-titanium alloys cannot be heat treated, but they generally have good weldability; alpha-beta titanium alloys can be heat treated and most are also weldable; beta and quasi-beta titanium alloys are fully heat-treatable and generally It is also weldable.
Most common α-β titanium alloys used for turbine engines and airframe components are Ti-6Al-4V (Allvac Ti-6-4, abbreviated as Ti-6-4), and Ti-6-4 is used herein to represent ATI Allvac Corporation. The company produces titanium alloys, the company's main supplier of titanium alloys (and recently signed a $2.5 billion long-term supply contract for titanium alloys with Boeing). In addition, ATI Stellram, which has partnered with ATI Allvac to develop processing solutions, also uses these titanium alloy codes to describe processing requirements.
Ti-6-4 has excellent properties of strength, fracture toughness and fatigue resistance and can be made into a variety of product forms. Annealed Ti-6-4 can be widely used in structural parts. Through the small changes in chemical composition and different thermo-mechanical processing techniques, Ti-6-4 can produce a wide variety of different parts.

Titanium alloy Ti-5Al-5V-5Mo-3Cr

Ti-5Al-5V-5Mo-3Cr (referred to as Ti-5-5-5-3) is a novel titanium alloy with considerable market influence. Compared with beta titanium alloys and alpha-beta titanium alloys, this quasi-beta titanium alloy can provide the required fatigue fracture toughness in aircraft component applications requiring higher tensile strength.
Compared with conventional titanium alloys (such as Ti-6-4 and Ti-10-2-3), Ti-5-5-5-3 has a forgeable complex shape and ultimate tensile strength after heat treatment can reach 180 ksi ( Thousands of pounds per square inch and other properties make it the most promising material for aircraft advanced components and landing gear.
Ti-5-5-5-3 can obtain excellent mechanical properties by carrying out a dissolution heat treatment below the β transformation temperature or annealing treatment above the β transformation temperature while properly controlling the grain size and precipitation in the microstructure. The beta transition temperature is the specific temperature of the composition at which the alloy transforms from an alpha-beta microstructure to an overall beta microstructure.
Changes in chemical properties and microstructures make titanium alloys available in a wide range of performance combinations and are therefore widely used in aerospace components. The processing difficulty of Ti-5-5-5-3 is approximately 30% higher than that of Ti-6-4, so the parts manufacturers applying this new alloy are working hard to develop without shortening the tool life and without prolonging the production cycle. The corresponding processing technology.
Material hardness is a key factor when processing titanium alloys. If the hardness value is too low (<38HRC, the titanium alloy will be sticky and the cutting edge will be prone to built-up edge, while the titanium alloy with higher hardness (>38HRC) will wear the cutting tool and cause the cutting edge to wear. It is important to choose the correct machining speed, feed amount, and cutting tool.

Requirements for cutting tools

In order to meet the requirements of production costs, processing quality and on-time delivery, new workpiece materials and parts design have increased pressure on aerospace component manufacturers. The processing of these new materials has changed the requirements for cutting tools. Increased metal removal rates, tool life, product quality and predictable tool life without damage are critical for efficient and safe machining. "Difficult-to-machine" is a relative concept, and efficient productivity can also be achieved through the correct combination of cutting tools and machining parameters.
When machining aircraft grade titanium alloy workpieces, cutting tool manufacturers control cutting heat generated by the tool-tool interface by increasing matrix density, designing special tool geometries, using precise cutting edge grinding technology, and developing new coating technologies The method greatly improves the tool performance.
One of the important characteristics of titanium alloys in milling is poor thermal conductivity. Due to the high strength and low thermal conductivity of titanium alloy materials, extremely high cutting heat is generated during processing (up to 1200°C if not controlled). Heat does not come out of the chips or is absorbed by the workpiece, but accumulates on the cutting edge. Such high heat will greatly shorten the tool life.
With special processing techniques, it is possible to increase the tool performance and life (using the correct processing technology to control the temperature, the temperature can be reduced to 250 ~ 300 °C).

Reduce heat generation

Reducing the radial and axial engagement of the tool with the workpiece can control the generation of cutting heat. For titanium alloys, the adjustment time for speed, feed, radial and axial joints is very short before the build-up of the product occurs due to overheating. In order to achieve the proper tool life, only 15% of the "welding arc length" is required for processing titanium alloys, compared to 50% to 100% of the arc length when processing ordinary steels. Reducing the contact arc length can increase the cutting speed and increase the metal removal rate without losing the tool life.
Cutting tools with a 45° cutting angle or thinner chips can increase the contact length between the cutting edge of the tool and the chip, thereby reducing local high temperatures and extending the life of the cutting edge. It also allows higher cutting speeds.

Blade geometry design

When cutting titanium alloys, the use of peripheral grinding blades is essential to minimize cutting pressure and friction with the surface being machined. The blade geometry must use a positive angle, but this is not enough to ensure optimal performance. If a small initial angle of high strength is used to enhance the first part of the cutting edge, then using a larger secondary angle (to obtain a larger positive chamfer) is best for enhancing blade resistance and tool life. Geometric design. In addition, a slight passivation also helps protect the cutting edge, but passivation dimensions must be coordinated with the cutting process and maintain tight tolerances. When machining titanium alloys, it is necessary to use a sharp cutting edge to cut the material, but sharp cutting edges can cause chipping and shorten tool life. Appropriate passivation protects the cutting edge and prevents premature chipping. Correct blade geometries reduce stress and pressure on the tool material, allowing the tool to achieve longer life and increase machining efficiency.
The cutting angles of the cutter body and the blade must be positive angles in order to achieve progressive cutting effects and avoid impacts on the entire cutting edge during cutting without achieving the desired shearing effect. If you do not do this, the workpiece structure may be deformed, making machining impossible.

Pocket Milling and Helical Interpolation Milling

In case of pocket milling and spiral interpolation milling, internal cooling tools must be used. If possible, constant pressure coolant should be used, which is especially important for deep pocket or deep hole machining.
When machining deep cavities, the use of high-density cemented carbide extension tools with modular cutting heads can increase rigidity and reduce deflection and give optimum machining results.
The function of the coolant is to remove the chips from the cutting area and avoid secondary cutting that may cause early failure of the tool. At the same time, the cooling liquid also helps to reduce the temperature of the cutting edge, reduce geometric distortion of the workpiece, and prolong tool life.
Spiral interpolation of milling holes with a milling cutter can reduce the use of other tools (such as drills) in the tool magazine. A diameter cutter can be used to machine different sizes of holes.
As the application of titanium alloys in the aerospace industry continues to grow, cutting technologies that support efficient machining of titanium alloys are also evolving. Due to the large demand for processing capability of titanium alloy parts, those workshops or manufacturers that use the most efficient processing technology will benefit first.

Internal integration creates new solutions

Allegheny Technologies is a multi-domain manufacturer whose business unit includes both metal smelting and metal cutting. The combination of these two fields has given the company an advantage in developing new processing methods for advanced materials such as titanium alloys.
ATI Stellram Corporation is a business unit of ATI Metalworking Products, an Allegheny Technologies company, which is responsible for the process performance testing of all new materials developed by ATI Allvac to determine the best blade design, tool geometry, substrate and coating structure As well as cutting parameters, these new materials can be cost-effectively processed before being marketed and sold. In addition, as a representative of Allvac, Stellram is a major aerospace manufacturing company and a top supplier of aerospace machinery parts, which can meet the common needs of both workpiece materials and cutting tools.
The comprehensive understanding of the inherent structure of the material enabled ATI Stellram to have an advantage in designing a unique tool matrix formulation. One of the results was the X-Grade technology. ATI Stellram stated that this technology has proven to be a reliable tool for processing difficult-to-machine materials. Program. By researching and developing X-Grade technology, a new carbide grade has been produced that can efficiently cut difficult-to-machine materials with extremely high metal removal rates under unstable processing conditions.

X-Grade Blade Technology (Matrix and Coatings)

The X-Grade inserts use a ruthenium/cobalt alloy matrix that resists the generation and expansion of hot cracks and provides high metal removal rates. The matrix has a strong crystal-binding matrix structure, which improves the toughness of the cutting edge. According to ATI Stellram, the matrix material, combined with new tool geometries and coatings, provides a superior tool combination for machining aerospace alloys. Using X-Grade inserts can achieve: 1 metal removal rate increased by 1; 2 tool life increased to 3 times; 3 processing surface finish increased by 30%.
Available X-Grade inserts include 3 grades (X400, X500 and X700), each designed for specific difficult-to-cut applications. They can use standard blade types and can be mounted in the blade pocket of a standard body. However, ATI Stellram said that the best solution is to use specially designed tools to optimize the performance of X-Grade blades. The sipe design of these tools allows for maximum chip removal, enhanced slot geometry, and optimal cooling. The two tools in this series include: 17710VR anti-rotation button cutters: with round inserts and a patented locking indexing system that prevents the blade from shifting at high feedrate cuttings; 27792VX high-feed milling cutter: with conventional tools In comparison, the metal removal rate can be doubled. In addition to high-feed surface milling, the 7792VX series tools can also be used for milling pockets, slot milling, and plunge milling. Since the cutting forces are directed axially into the spindle, spindle friction can be reduced and cutting stability can be improved.

Aviation Titanium Alloy Parts Processing Case Study

The following are two examples of machining aerospace Titanium Parts using ATI Stellram tools and X-Grade inserts.
(1) Processing Example 1
Parts Machined: Titanium Alloy Covers (Military)
Workpiece material: Ti-6Al-4V (Allvac Ti-6-4 alloy)
Workpiece size: 110"×18"
Processing Description: The ATI Stellram 7792VX high feed milling cutter with XDLT-D41 indexable inserts is used for machining. The life of the roughing tool reaches 156 minutes.
Milling cutter: C7792VXD12-A3.00Z5R; Number of burrs: 5
Blade: XDLT120508ER-D41; Grade: X500 (Designed with X-Grade Technology)
Axial cutting depth ap: 0.080"
Radial cutting width ae: 2.100"
Cutting speed vc:131sfm
Feed per tooth fz: 0.023 ipt
Feed rate: 19.2 ipm
Tool life: 156 minutes/index (4 times per blade)
(2) Processing Example 2
Parts Machined: Turbine Blades for Military Aircraft (New Applications)
Workpiece Material: Full Titanium Alloy Blade Size: 23.6" x 11.8"
Processing Description: Propeller blades are machined with ATI Stellram 7710VR milling cutters equipped with anti-rotation blades. The life of roughing tools reaches 110 minutes.
Milling cutter: C7710VR12-A2.00Z5R; Number of burrs: 5
Blade: RPHT1204MOE-421-X4; Grade: X700 (Designed with X-Grade Technology)
Axial cutting depth ap: 0.080" to 0.100"
Radial cutting width ae: 0.800" to 1.37"
Cutting speed vc:265sfm
Feed per tooth fz:0.0086 ipt
Feed rate: 21.8 ipm
Tool life: 110 minutes/index (4 times per blade)